#include <bits/stdc++.h>
using namespace std;
#define PI 3.1415926
#define   SQUARE(x) ((x)*(x))
//double SQUARE(double x) { return x*x;}

int main()
{
	int a1 ,a2;
	int n;
	scanf("%d%d%d",  &a1, &a2, &n);
  printf("%d", a1+(a2-a1)*(n-1));

  return 0;
}
